Q.Which of the following is true about Rh factor in the offspring of a parental combination DdXDd (both Rh positive)?

Alongside the ABO antigens, human red cells carry a separate antigen system, the Rh factor — specifically the 'D' antigen — discovered by Landsteiner and Wiener (1940) in rhesus monkey blood and soon after in humans. A person carrying the D antigen is Rh-positive; a person lacking it is Rh-negative. Unlike ABO antibodies, anti-D antibodies are not present naturally; an Rh-negative person only develops them after their immune system is actually exposed to Rh-positive blood.

Two genetic models explain Rh inheritance. The Fisher-Race hypothesis (in wider modern use) holds that Rh type is controlled by three closely linked gene pairs — C/c, D/d, E/e — at three adjacent loci on one chromosome; any genotype with at least one dominant D allele is Rh-positive, and only the double-recessive dd genotype is Rh-negative. The Wiener hypothesis instead proposes a single locus with eight possible alleles (R1, R2, R0, Rz, r, r1, r11, ry), where again a dominant 'R'-type …